PhysicalDisk perfmon counters versus LogicalDisk counters

Hi,
We have a Physical disk which is partitioned into 4 drives,D,E,F,J.We are using Windows server 2003. I am monitoring individual logical disk idle time and physical disk idle time. These are a day's average values

Counter	                                  Value
LogicalDisk(D:)\%idle time	        84.92676725
LogicalDisk(E:)\%idle time	        87.60788474
LogicalDisk(F:)\%idle time	        94.81640539
LogicalDisk(J:)\%idle time	        98.49689629
PhysicalDisk(1 D: E: F: J:)\%idle time	74.4477885

I noticed PhysicalDisk valueis much lower than average of LogicalDisk values,even though LogicalDisk values look good. PhysicalDisk value is suppose to be average of LogicalDisk values right? Why this variation caused?

Thank you